Field of the Invention
This invention relates generally to a steel alloy that provides a unique combination of strength, toughness, and fatigue life. More particularly, the invention relates to a useful article made from the steel as well as a method of making the article.
Description of the Related Art
Directional drilling of oil wells often require the use of mud motors. A mud motor (or drilling motor) is a progressive cavity positive displacement pump (PCPD) placed in the drill string to provide additional power to the bit while drilling. The PCPD pump uses drilling fluid (commonly referred to as drilling mud, or just mud) to create eccentric motion in the power section of the motor which is transferred as concentric power to the drill bit by way of the mud motor shaft and a constant velocity joint. Because the drill bit encounters deposits of varying hardness and strength during the drilling operation, the transfer of the eccentric motion as concentric power through the shaft requires a strong shaft material that has high impact toughness as well as good rotating bending fatigue life. The current material of choice is the 4330V alloy that has been known to provide a yield strength (KS) of about 150 ksi (1,034 MPa) and a Charpy V-notch impact energy (CVN IE) of about 40 ft-lbs. (54.2 J) at room temperature.
Up until recently the 4330V shaft material has been acceptable. Now with drilling of deeper wells into different deposits, such as shale, a need has arisen for a stronger shaft material with better toughness than provided by the 4330V alloy.